1. Field of the Invention
The present invention relates to a fast zoom lens system which is suitable for a single lens reflex (SLR) camera, and particularly suitable for adigital SLR camera.
2. Description of the Prior Art
A conventional fast zoom lens system of a negative-lead type, in which a lens group having a negative refractive power is positioned at the most object-side therein, has been advantageous in achieving higher quality photo-images. Therefore varieties of such a negative-lead type fast zoom lens system have been proposed.
For example, in Japanese Unexamined Patent Publication (hereinafter, JUPP) No. 2004-198529, JUPP No. 2003-84198 and JUPP No. 2005-49843, a fast zoom lens system of four lens-groups arrangement, i.e., a lens group with a negative refractive power, a lens group with appositive refractive power, a lens group with a negative refractive power and a lens group with a positive refractive power, in this order from the object, have been proposed.
Alternatively, a conventional positive-lead type fast zoom lens systems of four lens-groups arrangement, i.e., a lens group with a positive refractive power, a lens group with a negative refractive power, a lens group with apositive refractive power and a lens group with a positive refractive power, in this order from the object, has been proposed in, e.g., JUPP No. 2004-101739. A fast zoom lens system of this positive-lead type has been advantageous for miniaturizing both overall length and lens diameters of the fast zoom lens system.
A negative-lead type fast zoom lens system, however, tends to be heavier. On the other hand, a positive-lead type fast zoom lens system is advantageous for miniaturization, while it is difficult for a positive-lead type fast zoom lens system to secure a sufficient back focal distance.
In a digital SLR camera, the size of an image sensor thereof is smaller than the film-frame size of a silver-halide film SLR camera, so that the focal length with respect to a predetermined angle-of-view becomes shorter.
Therefore if an attempt is made to make a conventional lens system designed for a silver-halide film SLR camera compatible with the digital SLR camera, it is necessary to make the flange-back of the digital SLR camera longer so as to be substantially the same as that of a silver-halide film SLR camera. Consequently, a longer back focal distance with respect to a shorter focal length is necessary in order to achieve a lens system of a wider-angle (shorter focal length).